I created a ETH trust wallet on my phone for someone and he has now imported the trust wallet to his phone. Is there any way to change the 12 word phrase or somehow give the sole control to the wallet to him? He is concerned about security as the wallet still shows on my phone and I have the 12 word phrase.
Private keys are derived from the mnemonic phrase directly, so if you use a different mnemonic phrase (different words), you'll get different private keys, and thus addresses.
The best way to approach this is to generate a new mnemonic phrase and move (send) all funds from the old addresses to the new ones. They should generate the new phrase themselves, so only they have access to this phrase.